The present invention relates to a process for etching a ferroelectric film disposed on a substrate. More specifically, the present invention relates to a process and solution for wet etching a perovskite material disposed on a silicon substrate.
Ferroelectric films have been actively utilized for electronic and optoelectronic applications such as memory devices, transistors and optical devices. Many perovskite ferroelectric materials are notable as advanced dynamic random access memory (xe2x80x9cDRAMxe2x80x9d) capacitors for the one gigabit and beyond range. These perovskite materials include barium titanate, BaTiO3 (xe2x80x9cBTOxe2x80x9d), strontium titanate, SrTiO3 (xe2x80x9cSTOxe2x80x9d), and their solid solutions, barium strontium titanate, (Ba,Sr)TiO3 (xe2x80x9cBSTOxe2x80x9d). BSTO is especially useful in electrical and optoelectrical devices because of its lower leakage current and better aging properties.
A common structure employed in DRAM devices is a stacked cell structure, in which a BSTO capacitor is connected to a pass transistor via a poly silicon contact plug. Platinum is generally used as the bottom electrode for the capacitor. The platinum is placed on a silicon oxide or silicon nitride substrate and a contact plug. Platinum is utilized since the bottom electrode must endure a high temperature oxygen environment during the deposition of the perovskite material. Additionally, to avoid an undesirable reaction between the platinum electrode and the silicon substrate, one or more diffusion barrier layers may be juxtaposed between the platinum electrode layer and the silicon oxide or silicon nitride substrate and the contact plug. The bottom electrode is patterned and a ferroelectric perovskite material such as BSTO is deposited onto the bottom electrode and the field region that does not contain bottom platinum. Then a top electrode is deposited onto the ferroelectric perovskite material. A cover and mask layer may be placed onto the top electrode layer. Such a stacked cell structure may be fabricated using established patterning technologies. Such patterning technologies involve the etching of various layers of the stacked cell structure to expose other layers for proper functioning of the electronic device. Dry or wet etching techniques may be employed to fabricate the finished stacked cell structure.
One known etchant for BSTO is hydrogen fluoride. Although hydrogen fluoride is an adequate etchant for BSTO, it has the detrimental effect of etching exposed silicon and oxide surfaces. Thus, the use of hydrogen fluoride to etch a BSTO layer of a stacked cell structure would result in other layers being etched thereby possibly damaging the electronic device.
Another etchant that may attack the titanate bonds of a perovskite lattice is hydrogen peroxide. The hydrogen peroxide forms a strong complex with the titanium. However, the etch rate of BSTO in hydrogen peroxide is extremely slow, well below even the lowest possible commercial production rate. Thus, the use of hydrogen peroxide would substantially increase the production costs of the electronic device.
Other etchants include hydrochloric acid, see U.S. Pat. Nos. 5,374,330 and 5,356,516, illuminated with radiation. Illumination is required for etching titanates (such as BSTO) in the HCl acid liquid ambient, which increases production costs.
The present invention is directed to solving one or more of the problems discussed above in a novel manner.
The present invention provides a solution to the problems of the prior art associated with the fabrication of DRAM devices and other electronic devices. The present invention is able to accomplish this by providing a process and solution for wet etching of ferroelectric films at a commercially acceptable rate of production.
One aspect of the present invention is a process for etching a perovskite ferroelectric material. The process includes providing a substrate having a ferroelectric film. The ferroelectric film is composed of a perovskite material. The process also includes subjecting the ferroelectric film to a mixture of hydrogen peroxide and an acid. The process also includes heating the mixture to a temperature between 30 and 90 degrees Celsius.
The perovskite material of the process may be barium strontium titanate, barium titanate, strontium titanate, calcium titanate, lead titanate, lead lanthanum zirconium titanate or lead zirconium titanate. The acid of the process may be sulfuric acid, hydrochloric acid, phosphoric acid, acetic acid or the like. The mixture may be composed of between approximately 20% to approximately 95% of hydrogen peroxide, 5% to 80% acid, and 0% to 75% deionized water.
More specifically, the mixture of the process may be composed of approximately 45 percent hydrogen peroxide, approximately 10 percent acid and approximately 45 percent deionized water.
The step of subjecting the ferroelectric film to the mixture may include placing the substrate with the ferroelectric film disposed thereon into a solution of the mixture for a predetermined time period to effect the necessary etching of the ferroelectric film. The predetermined time period may be between 30 and 1000 seconds or longer.
Another aspect of the present invention is a process for etching a perovskite ferroelectric material which includes providing a substrate having a ferroelectric film composed of a barium strontium titanate, barium titanate, strontium titanate or a lead titanate. The substrate also has a silicon oxide or silicon nitride component and an electrode. The process also includes subjecting the substrate having the ferroelectric film to a solution of hydrogen peroxide and an acid. The process also includes heating the solution to a temperature between 25 and 90 degrees Celsius.
The electrode of the process may be composed of platinum. The acid of the process may be sulfuric acid. The solution of the process may be composed of between approximately 20% to approximately 95% of hydrogen peroxide, 5% to 80% acid, and 0% to 75% deionized water. The solution of the process may be heated to a temperature between 30 and 70 degrees Celsius. The step of subjecting the substrate having a ferroelectric film to the solution may include placing the substrate having the ferroelectric film into a bath of the solution for a predetermined time period to effect the necessary etching of the ferroelectric film. The predetermined time period may be between 30 and 240 seconds.
Another aspect of the present invention is in a solution for a wet etch process for selective removal of a titanium based perovskite material. The solution consists essentially of hydrogen peroxide which forms a complex with the titanium of the titanium based perovskite material, an acid capable of stabilizing the complex formed between the hydrogen peroxide and the titanium, and optionally deionized water.
The solution may be composed of between approximately 20% to approximately 95% of hydrogen peroxide, 5% to 80% acid, and 0% to 75% deionized water. The acid of the solution may be sulfuric acid. The solution may be heated to a temperature of between 25 and 90 degrees Celsius. The titanium based perovskite material of the solution may be barium strontium titanate, barium titanate, strontium titanate or lead titanate. The solution may etch silicon oxide at a rate of less than 10 Angstroms per minute at a temperature of between 30 and 70 degrees Celsius.
Having briefly described the present invention, the above and further objects, features and advantages thereof will be recognized by those skilled in the pertinent art from the following detailed description of the invention when taken in conjunction with the accompanying drawings.